This invention relates to a motor control circuit for induction motors, and more particularly a motor control for a controlled current induction motor drive system capable of stable four quadrant operation.
It is well known that under certain operating conditions a conventional voltage and frequency controlled induction machine operating either as a motor or generator can exhibit self-sustaining oscillations about a steady-state operation point. These instabilities are not a result of load or disturbance torques but are directly related to the machine parameters and operating constraints. Similar stability problems occur in current and frequency controlled induction motor drives, and it is necessary to add feedback control loops to achieve all operating points. Motor control circuits for such controlled current drive systems use a current controlled inverter in which the output current waveshape and frequency are controlled rather than the output voltage and its frequency as in the usual inverter.
Controlled current induction motor drives with feedback controls for four-quadrant operation have been disclosed in the prior art, such as in the article entitled "Static Induction Motor Drive: An Ideal Drive for the Paper Industry" by Larry D. Beer, IEEE Conference Record of 1971 Annual Pulp and Paper Industry Technical Conference, Apr. 21-23, 1971; and also in U.S. Pat. Nos. 3,863,121 or 3,863,122 granted on Jan. 28, 1975 to Charles E. Rettig and both assigned to Litton Industrial Products, Inc. All three of these describe essentially the same motor control technique in which the speed error commands the actual value of slip frequency and slip frequency in turn regulates the commanded value of stator current. Additionally, tachometer feedback of the rotor speed is employed to constrain the inverter output or stator current frequency to be the sum of the commanded slip frequency and sensed rotor frequency. With such an arrangement, unstable operation can result when the rectifier voltage determining the stator current magnitude is constant and at its maximum, and no longer is able to respond to changes in the stator current command. The present invention is directed to an improved motor controller to obtain a more stable current controlled induction motor drive system capable of four quadrant operation under optimized rated flux conditions.